Fiber Bragg grating sensors (FBG sensors) have demonstrated themselves to be attractive devices for sensing temperature and strain along an optical fiber. Variations in the spectral response of the grating result from period changes in the Bragg grating due to strains or temperature variations that are experienced by the in-situ optical fiber. These FBG sensors offer important advantages over other sensor technologies because of their electrically passive operation, electromagnetic interference (EMI) immunity, high sensitivity and multiplexing capabilities. Fiber Bragg gratings are simple, intrinsic sensing elements which traditionally have been UV photo-inscribed into photosensitive Ge-doped silica fiber. Each FBG sensor has a characteristic retro-reflective Bragg resonance or Bragg wavelength, which is dependent upon the periodicity of the grating photo-inscribed within the fiber and the effective refractive index difference in the grating regions of the optical fiber. The FBG sensors can then easily be multiplexed in a serial fashion along a length of single fiber. When embedded into composite materials, By continuing the exposure and grating inscription of the fiber in the type I regime such that the index modulation becomes about >3×10−3, the threshold for type II grating formation is reduced in a continuous fashion until it traverses the grating inscription intensity, which in this instance was ˜1.5×1013 W/cm2. The index modulation value is likely directly inversely proportional to the type II threshold intensity value.
Optical fibers with an array of FBG sensors allow for distributed measurements of load, strain, temperature and vibration of the material creating what has is commonly referred to as “smart structures” where the health and integrity of the structure is monitored on a real-time basis.
Typically fiber Bragg gratings are generated by exposing the UV-photosensitive core of a germanium doped silica core optical fiber to a spatially modulated UV laser beam in order to create permanent refractive index changes in the fiber core. Such a spatially modulated UV beam can be created by using a two-beam interference technique as disclosed in U.S. Pat. No. 4,807,950 by Glenn et al. or by using a phase mask as disclosed in U.S. Pat. No. 5,367,588 by Hill et al. The techniques taught by Glenn and Hill result in gratings that are typically referred to as Type I gratings.
A limitation of the prior-art UV-induced Type I fiber Bragg gratings, especially for high temperature sensor applications is that operation of the sensor at elevated temperatures results in the erasure or annealing of the UV-induced color centers and densification which are responsible for the induced index change of the grating. In fact, at temperatures approaching the glass transition temperature of the fiber, which for silica is approximately 1000° C., total erasure of the induced index modulation results. The fiber also is modified at such high temperatures making it brittle with diffusion of the core material into the cladding. The fiber can easily be deformed by its own weight.
Another method for creating permanent photoretractive index changes in glasses employs the use of intense UV beams with fluences or energy/unit-area per laser pulse densities that approach those required to produce macroscopic damage of the glass. Askins et al. in U.S. Pat. No. 5,400,422 teach a method for producing permanent photoretractive index changes in the photosensitive cores of Ge-doped optical fibers with single high intensity UV laser pulses. Such Bragg gratings resulting from macroscopic damage to the glass optical fiber are typically referred to as Type II gratings. The high intensity portions of the interference fringes created by two crossed UV beams split from a single UV beam create localized damage at the core-cladding interface within the fiber. Because the process for inducing index change is one of structural change due to localized physical damage to the glass, rather than due to UV photoinduced color center formation, the induced index change is more robust and does not decrease with elevated temperature. In fact Askins et al. disclose that gratings produced in this way cannot be removed by annealing until the fiber or waveguide approaches the material's glass transition temperature. The drawback of this approach for induction of index change is that the Bragg gratings produced in this fashion have relatively low refractive index modulations (Δn=10−4) and are mechanically weak since the effective refractive index change results from periodic localized damage at the core-cladding interface. When the pulse duration is long (>a few tens of picoseconds) laser-excited electrons can transfer energy to the surrounding lattice faster than the thermal diffusion of the material can remove the energy resulting in damage. If the laser pulse continues to feed energy into the damage site, the damage can propagate beyond the irradiated zone. For damage grating structures written with long laser pulse durations greater than a few tens of picoseconds, the spectral quality of the resulting Bragg grating is often poor.
Another method for creating permanent photoretractive index changes in optical fiber employs the use of the process of “hydrogen-loading,” as taught by Atkins et al. in U.S. Pat. No. 5,287,427, combined with UV-laser exposure of optical fiber that is manufactured with a core that is co-doped with fluorine. Subsequent to the UV exposure the fiber then undergoes a thermal post treatment at 1000° C. in order to induce a chemical composition grating as taught by Fokine in U.S. Pat. No. 6,334,018. As with the technique taught by Askins et al., the technique taught by Fokine also has the drawback that the induced index change of the Bragg gratings produced in this fashion have relatively low refractive index modulations (Δn=10−4).
The fabrication of high temperature stable Bragg gratings using infrared ultrafast radiation and a phase mask, as taught by Mihailov et al in U.S. Pat. No. 6,993,221 results in high temperature stable Bragg gratings with very high index modulations (Δn>10−3). As shown by Smelser et al. Opt. Express., vol. 13, pp. 5377-5386, 2005, laser beam intensities greater than 4×1013 W/cm2 at the surface of the optical fiber result in the formation of thermally stable Bragg gratings similar to Type II UV-induced gratings, but with much higher index modulations. In the case of gratings fabricated using the techniques taught in U.S. Pat. No. 6,993,221, the index modulation results from a threshold type process of multiphoton absorption/ionization that results in plasma formation and the possible creation of microvoids. Although strong gratings can be formed using the approach taught in U.S. Pat. No. 6,993,221, the resulting gratings suffer from high scattering loss making it difficult to concatenate a large number of Bragg grating sensors into a sensor array on a single length of optical fiber. The threshold nature of the process also makes it more difficult to tailor the induced index profile of the grating in terms of its apodization, reflectivity and reflection bandwidth. Using the technique taught by Mihailov et al in U.S. Pat. No. 7,031,571, Smelser et al. showed that very high index modulations (Δn>10−3) could be created with laser beam intensities lower than 4×1013 W/cm2 that did not possess high scattering loss however these gratings were not high temperature stable and the index modulation likely arising from color center formation and material densification, erased at temperatures >800° C.
The inventors have recently discovered that induction of extremely large index modulations (Δn>3×10−3) in Germanium doped telecommunication optical fiber using the technique disclosed by Smelser et al. in Opt. Letters., vol. 29, pp. 2127-2129, 2004 has the beneficial effect of being stable up to 1000° C.
The thermal stability of the refractive index change that is generated using prolonged laser exposures consistent with a type I grating formation process may be caused by the extremely large index modulations that are generated. In one aspect of the present invention the inventors arrive at these high levels of index modulation for example 3×10−3, through the process of hydrogen loading of Ge-doped silica fibers coupled with femtosecond infrared laser exposure through a phase mask.
In accordance with another aspect of the invention, an extremely high index modulation such as 3×10−3 can be generated by femtosecond infrared laser exposure consistent with type I grating formation in optical fibers or waveguides which have high concentrations of core co-doping with for example Germanium. In Grobnic et al Photon. Technol. Lett. vol. 20, no. 12, pp. 973-975, 2008, high NA, high Ge-doped core fibers are more photosensitive to femtosecond IR radiation than standard telecom (low Ge-core concentration) fibers. It is possible that large index changes induced in the high Ge-doped fibers have improved thermal stability. We believe that such high refractive index changes for example 3×10−3 lower the intensity threshold sufficiently to yield a stable grating with minimal scatter.
It is an object of this invention to overcome the aforementioned limitations within the prior art systems for fabrication of high temperature FBG sensors by inducing large refractive index modulations in silica-based optical fibers that are relatively stable up to 1000° C. and which do not suffer from high scattering or insertion loss and.